1. Field of the Invention
The present invention relates to a signal control system, and more particularly to a control system for a digital signal on a speech path in a time-division switching system.
2. Description of the Prior Art
In a space-division switching system handling an analog signal, even if an originating terminal is connected, via a digital transmission line, with a terminating terminal, it is required that a loss for retaining moderate speech quality be imparted by the terminal equipment to a repeater line. In a time-division switching system handling a digital signal, when a digital transmission line is used, no terminating terminal equipment is needed because the originating terminal can be interfaced directly with the digital transmission line. Accordingly, it is necessary to connect a loss pad switch for varying the loss to the digital transmission line in the originating terminal; because the loss to be inserted differs with the kinds of connections and terminating terminals, and thus various insertion losses (for example, 3 dB, 5 dB, 6 dB, etc) must be created. A digital pad for inserting these different losses has been proposed which is to be inserted into a highway.
In the time-division switching system, switch components can be made of semiconductor parts to permit rapid switching; therefore, connection, disconnection, switching and like operations for the speech path can be performed instantaneously by directly controlling a time-division speech network formed of electronic parts such as memories, gates and so forth.
When a time switch with a random write-sequential read arrangement is included on the backward side of the time-division speech network, the setting and disconnection of the speech path takes place in the manner described below.
When using time switch, the speech path is set up by reading out data written in a time switch control memory by a common control unit, writing speech information in the speech memory using the read-out data as a write address, and reading out the written speech information from the speech memory onto the backward highway using the output data from a channel counter as a read address. The disconnection of the speech path is performed by erasing the address data written in the time switch control memory. In short, when the address data in the time switch control memory is erased, speech information is no longer written into the speech memory and the transmission from speech memory is stopped, thus cutting off the speech path. In this case, the read addresses from the channel counter continue, thus, continuing the read out of speech information from the speech memory to the forward highway.
In the above case, the cutting off of the speech path is accompanied by the following problems:
After cutting off of the speech path, new speech information is not written into the speech memory of the time switch at the address assigned to the speech path, so that the speech information stored before cutting off the speech path remains in the speech memory. As a result, constant data (the residual data) are continuously sent out on a channel of the output side. When the data are decoded by a decoding circuit, a DC current having a level dependent on the residual data is provided at the output of the decoding circuit.
Accordingly, when connecting to the aforesaid speech network a signal processing unit susceptible to interference by the DC signal in the output of the decoding circuit, for example, a conference trunk, there must be provided means for cutting off the DC signal for the following reason. In the conference trunk of the time-division switching system, digital speech signals inputted from a plurality of input terminals are decoded by a decoding circuit into analog signals and then mixed together, thereafter the mixed signal is distributed to subscribers taking part in a conference; therefore, a DC signal with a high level may sometimes be applied to the conference trunk by the decoding circuit having an input terminal connected to a disconnected speech path, thus interfering with the conferenced speech.
Further, when the originating terminal is connected with a digital repeater line, the residual data are continuously sent out to the terminating terminal without being decoded, so that according to the pattern of the residual data, for example, when the data are all "0," clock pulses cannot be reconstructed in the terminating terminal, thus making signal transmission impossible.
Moreover, when changing the connection of a speech path of the speech network, a signal generated by the residual data appears in the signal to be newly transmitted and according to the pattern of the residual data, a high degree of discontinuity is introduced into the signal and the discontinuity generating noise.
The above problems are created when the time switch of the random write-sequential read construction is included on the backward side of the time-division speech network.
In the prior art, a signalling tone is obtained by a continuous signal tone being intermittently applied, for example, a busy tone is generated by ON-OFF control of a continuous signal source, however, this ON-OFF control can be achieved in the time-division speech network. When the time-division speech network interrupts the signal tone, however, exactly the same problems are created as described above.
In the transmission of a howler tone, it is general practice in the prior art to connect a howler trunk to a telephone set having a permanent call and gradually increase the sending level of the howler tone in the howler trunk. To perform this, each howler trunk must be equipped with means for gradually increasing the sending level, and it is impossible to simultaneously send out a howler tone from one howler trunk to a plurality of telephone sets since the howler sending operation starts at a particular moment for each telephone set.
Furthermore, a multi-frequency signal receiver correctly receives only when the signal receiving level lies within a certain range, and to ensure correct reception the receiver is tested by sending thereto signals having different receiving levels. In the time-division switching system, in order to vary the receiving level of the multi-frequency signal, it is customary in the prior art to connect a multi-frequency signal transmitter via a time-division speech network to a piece of test equipment and to connect the test equipment via the time-division speech network to the multi-frequency signal receiver under test. Thus forming a test circuit from the multi-frequency transmitter to the multi-frequency receiver under test, wherein various losses are inserted by the test equipment. Accordingly, such special test equipment as mentioned above is required in the prior art.
As described above, the prior art has the disadvantage that signal sending level changing means must be included in a tone sending device (for example, a howler trunk) or in the multi-frequency signal receiver test equipment so that a tone signal of varying sending level can be obtained.